More than likely he was a "snake oil salesman" since as an apothecary he would make and sell drugs. In those days, there were all kinds of drugs invented by pharmacists. Some of them actually worked, most did not. Scout says:

"Mindful of John Wesley's strictures on the use of many words in buying and selling, Simon made a pile practicing medicine ..." (chapter 1).

It sounds like Simon Finch was a good salesman. He was stingy, so he saved what he made. John Wesley was a Methodist theologian, so it appears that Simon Finch used Methodist principles, or his interpretation of them, to make himself a better salesman. He was successful enough to make the Finches an important name, and he is the founder of the “good family” that Aunt Alexandra tries to remind Scout of constantly.